This present disclosure is concerned with new processes employing multiple successive chemical reaction process steps and apparatus therefore, and especially to such processes and apparatus employing micro-agitation to facilitate the chemical reactions carried out in the successive steps. The present disclosure is concerned especially, but not exclusively, with such processes and apparatuses for the production of organic dicarboxylic acids, and even more especially with such processes and apparatus for the production of dimethyl terephthalate and terephthalic acid.